Fibers can be formed by electrospinning processes. In electrospinning, a portion, typically in the form of a droplet, of polymer solution or melt is elongated by a strong electrical field. The resulting fibers are collected as non-woven mats or as individual spun fibers. The fibers generally have a large surface to volume ratio and consequently are useful for various applications including filtration.
Most electrospinning processes are solution based, i.e., the fibers are generated from a solution of the polymer. Solution electrospinning requires the polymer to be highly diluted in the solvent in order to reduce viscosity to sufficient levels such that acceptable fibers can be formed. The high dilution, however, results in a low yield of fibers per unit of polymer solution processed, and consequently increases energy and production costs, and the costs of solvent recovery or disposal. In addition, there can be a significant environmental impact associated with use of high solvent levels. And for some conventional solution electrospinning polymer systems, it may take at least 15 minutes to an hour to obtain a visible (to the naked eye) coating on a collector.
In solution electrospinning, it is also known that smaller diameter fibers can be obtained by increasing the dilution of a conventional polymer in a solvent. Electrospinning of a dilute solution of the polymer fails when polymer concentration gets below its entanglement concentration because the polymer fails to form fibers from such a low concentration solution. These dilute solutions will instead be electrosprayed as polymer droplets. This drawback limits the minimum diameter of the fibers that can be obtained by solution electrospinning a conventional polymer. Further, low concentration solutions that can still be solution electrospun typically produce fibers having thicker structures (i.e., beads) scattered on them. If desired, some reduction of beading can be accomplished by adjusting process parameters. In some applications (e.g., filtration), the beading can provide a benefit.
Solution electrospinning systems that significantly mitigate the disadvantages of using high solvent concentrations are desirable. Fibers that exhibit favorable attributes or properties, such as uniformity and small size, are also desirable. Particularly desirable are systems that both address the disadvantages of high solvent concentrations and at the same time yield fibers with superior properties to those currently known. Some preferred embodiments of the present invention provide such systems. Higher production rates are desirable. Other preferred embodiments of the present invention provide processes for solution electrospinning small average diameter fibers from solutions at low polymer concentrations.